When a tissue experiences inflammation, its cells remember. Pinning proteins to its genetic material at the height of inflammation, the cells bookmark where they left off in their last tussle. Next exposure, inflammatory memory kicks in. The cells draw from prior experience to respond more efficiently, even to threats that they have not encountered before. Skin heals a wound faster if it was previously exposed to an irritant, such as a toxin or pathogen; immune cells can attack new viruses after a vaccine has taught them to recognize just one virus.